Semiconductor memory devices are typically classified into volatile memory devices and non-volatile memory devices. Volatile memory devices are subdivided into dynamic random access memories (DRAMs) and static random access memories (SRAMs). Non-volatile memory types include mask read-only memories (MROMs), programmable read-only memories (PROMs), erasable programmable read-only memories (EPROMs), and electrically erasable programmable read-only memories (EEPROMs). Additionally, flash EEPROMs are advantageous as mass storage devices because their integration density is high compared with conventional EEPROMs.
Non-volatile semiconductor memories have attained broad utilization due to an ability to retain data within a device, even after power has been suspended. EEPROMs are non-volatile semiconductor memories that posses these abilities and additionally are able to store data by electrically erasing and writing storage devices. This programming process can be repeated over hundreds and thousands of cycles.
Frequently, it would be convenient to be able to mix integrated circuit device types, such as EEPROMs with other memory devices or bipolar and MOSFET (BiCMOS) circuits onto a single integrated circuit chip. However, due to the inherently low breakdown voltage (approximately 10 volts or less) of the typical wells used in these devices and the need for a high programming voltage of a flash memory device (approximately 11 to 15 volts), there has been no simple and economical way to integrate these two device types into a single integrated circuit.